Adapt the line step of the scroll bars to the zoom factor.
For low zoom factors, scrolling with the mouse wheel is still precise, for higher zoom rates the steps get bigger.

This means the approx. the same amount of mouse wheel scrolling is required for to scroll the same span in the original picture, regardless of zoom rate.

For the overall relative speed, I added a factor of 2 which feels good, please report if scrolling steps are too big or too small.

(bug 145237, #4)